Let me start with saying Gin was never consider the strongest captain in any part of the manga.You won't find one scan anywhere in the manga stating Gin is the strongest or even on the level of the strongest.Gin was a child prodigy, nothing more.
Yamamoto...
Shunsui strength is clearly above Gin's.This is obvious seeing as he's took over as the head captain after Yamamoto died, and beat on the number 1 espada in Starrks without ever using his bankai.
Jushiro is also stronger than Gin, being one of the oldest captain and being trained by Yamamoto himself alone with Shunsui put Jushiro head above Gin.
Byakuya has shown way more skill and versatility than Gin.Thousands of blades versus one quick blade, I'll take the former.
Kenpachi - Gin could have being him during the time period when Ichigo went to the soul society to save Rukia, but not now.
Retsu Unohana...
Lastly, Gin's can't one shot anyone.No one gets one shotted in Bleach.And if an inexperience and scared Ichigo can block Gin's sword attacks a few times like he did in the Fake Karukura Town arc, every single captain in the Gotei 13 could surely handle such an attack with ease.
Also, to the person who said Kenpachi was stronger than Unohana, tried to read the manga before you make your claim.Unohana literally had to kill Kenpachi time after time and bring him back to life for Kenpachi to finally beat her.When Gin was alive it's obvious that Kenpachi wasn't on Unohana level.